Normalize the use of unicode strings

[tcalmant]

Currently, javaobj is loading Java strings as str objects. This is OK in Python 3, but wrong in Python 2.
All strings loaded from a Java serialized file should be of type unicode in Python 2.

Important note: this can break some existing code...

[tcalmant]

Fixed in commits:

• 4009115 Added a to_unicode() utility method
• 934afc9 JavaString inherits UNICODE_TYPE instead of str
• 2d47d22 Tested strings must be in unicode

Note: Now, JavaString inherits from unicode in Python 2 instead of str. Nothing changes in Python 3.